Seven African governments and the world's largest banks and construction firms meet in London today to plan the most powerful dam ever conceived - an $80bn (£40bn) hydro power project on the Congo river which, its supporters say, could double the amount of electricity available on the continent.

G8 and some African governments hope that the Grand Inga dam in the Democratic Republic of Congo will generate twice as much electricity as the world's current largest dam, the Three Gorges in China, and jump start industrial development on the continent, bringing electricity to hundreds of million of people.

But while governments and banks expect the dam to export electricity as far away as South Africa, Nigeria and Egypt, and even Europe and Israel, environment groups and local people warned that it could bypass the most needy and end up as Africa's most ruinous white elephant, consigning one of the poorest countries to mountainous debts.

The dam is being planned to exploit one of the largest major water falls by volume anywhere in the world - nine miles of rapids which lie 90 miles from the mouth of the Congo where the world's second largest river drops nearly 100 metres in just eight miles. Two hydroelectric plants, known as Inga 1 and Inga 2, were constructed near there in the 1970s and a third is planned, but Grand Inga is intended to dwarf them all. One feasibility study suggests the 40,000MW dam will be 150m high, and will harness 26,000 cubic metres of water a second, with more than 50 turbines each producing nearly as much power as a British nuclear reactor.

Grand Inga was proposed in the 1980s but never got beyond feasibility studies because of political turmoil in central Africa. But now it stands a chance, according to Gerald Doucet, secretary general of the World Energy Council thinktank, which is convening the London meeting.

"It is the greatest sustainable development project, offering Africa a unique chance for interdependence and prosperity," said Doucet. "It's much more feasible now than ever. There is a peace settlement in Congo, and economic and technical studies have all shown it is possible."

Grand Inga's prospects of being completed by 2022 are said to have risen significantly in the last year as countries, banks and private companies have found they can earn high returns from the emerging global carbon offset market and UN climate change credits. In return for investments in clean power, rich countries such as Britain hope to be able to offset their own greenhouse gas emissions against the renewable energy that dams such as Grand Inga would produce, and constructors are making windfall profits out of renewable energy projects in developing countries.

"The banks and the City of London see that Grand Inga is serious. The G8 countries are behind it because they can get UN clean development mechanism [CDM] credits to offset their emissions. Chinese, Brazilian and Canadian dam-building companies, as well as the World Bank, are all interested," said Doucet.

But advocacy groups said yesterday the plans ignored local people and could leave Congo with massive debts rather than a sustainable industrial base.

"The project would be a magnet for corruption in one of the world's least stable regions. Its enormous budget and large contracts could devolve Inga into a corruption-riddled white elephant. Inga will centralise a vast store of the region's electric and financial power, a development model that can foster tensions and civil wars," said Terri Hathaway, Africa campaigner with International Rivers, a watchdog group monitoring the project.

Hathaway said that the 94% of people in Congo DRC and the two in three Africans who have no electricity now were unlikely to benefit because the dam depends on exporting its electricity to existing centres of industry, especially in South Africa where there have been power shortages.

"As it stands, the project's electricity won't reach even a fraction of the continent's 500 million people not yet connected to the grid. Building a distribution network that would actually light up Africa would increase the project's cost exponentially. It would be very different if rural energy received the kind of commitment and attention now being lavished on Inga," she said.

Despite Congo having exported electricity for years from Inga 1 and Inga 2, access to electricity across the country is less than 6%, and in rural areas where nearly 70% of people live, it is only 1%.

"My village is 3km from Inga's power lines. They built a line almost 2,000km to the mines [in Katanga province] but in all of these years we have been left without electricity," said Simon Malanda, a community representative.

Doucet acknowledged that "there are huge social issues around Grand Inga that must be dealt with. Congo must benefit. If Congo is bypassed then the whole project fails," he said.